This invention relates to a copying machine of the diazo type. More particularly, the invention relates to a copying machine of the diazo type which comprises a transparent cylinder for printing a diazo copying sheet superposed with an original and a device for controlling the surface temperature of the transparent cylinder.
In conventional copying machines of the diazo type, if the temperature of the surface of a transparent cylinder for printing a copying sheet is maintained at a high level, it is possible to accelerate the printing speed and shorten the exposure time, with the result that the copying efficiency is enhanced. However, there is a defect since at too high of a temperature an original is damaged and the copying machine may be damaged.
In contrast, if the surface temperature of the transparent cylinder is maintained at a low level in order to prevent damage to an original or to the machine, the printing speed has to be reduced and the number of copies obtaind per unit time is lessened, with the result that the copying efficiency decreases.
Further, the surface temperature of the transparent cylinder is greatly influenced by the ambient temperature and this ambient temperature varies considerably depending on the season. Accordingly, in conventional copying machines of the diazo type, the surface temperature of the transparent cylinder is set in due consideration of the foregoing point. In general, a fan having such a cooling activity that the surface temperature of the transparent cylinder is maintained at 60.degree. to 70.degree.C. in summer (ambient temperature being about 35.degree.C.) is provided, such fan is continuously operated. However, when the same cooling condition is adopted for the whole year, it is impossible to obtain copies having the same image concentration and contrast throughout the year. In winter when the ambient temperature is low, a clear image cannot be obtained without reducing the printing speed. This defect has not yet been overcome.
The assignee of the instant application has proposed a process for preparing multi-colored copies which comprises the steps of (A) exposing to light a diazo type photosensitive material containing at least one diazonium salt, (B) superposing a layer of at least one thermovolatile or thermosubliming coupler on preselected areas of the diazo type photosensitive material and heating the assembly to effect transfer of the coupler, steps (A) and (B) being conducted simultaneously or in the sequence of (A) then (B), or (B) then (A) and developing the light-exposed and coupler-transferred photosensitive material in the presence of a coupler having a lower coupling rate under the development conditions than that of the volatile or subliming coupler, to obtain a multi-colored copy in which areas corresponding to preselected areas of an original have color different from that of other areas.
In a copying machine for use in practice of the above multi-color copying process, in order to effect heat transfer of a coupler, the surface temperature of a transparent cylinder should be maintained at a higher level than in ordinary diazo type copying machines, and a slight change in the temperature influences the color of a multi-color copied image. Accordingly, in order to obtain a good distinction among colors, it is necessary to maintain the surface temperature always at a constant level (for example, a temperature of 90.degree. to 110.degree.C.). In a copying machine provided with a perpetually operated fan having a cooling activity such as mentioned above, the surface temperature of a transparent cylinder varies depending on the change in the ambient temperature and a colored image having a desired good distinction among colors cannot be obtained. For these reasons, conventional diazo type copying machines cannot be conveniently employed for practising multi-color reproduction.
The assignee of the instant application has also proposed a copying machine applicable to either ordinary monochromatic reproduction or multi-color reproduction, in which a heat-generating member is provided in a transparent cylinder, whereby in the case of multi-color reproduction the quantity of heat generated by the heat-generating member is automatically controlled so as to maintain the surface temperature of the transparent cylinder at a constant level. However, this copying machine has a complicated structure and is expensive.